1. Technical Field
The invention relates to metallurgy and foundry engineering and particularly concerns a method of lining a metallurgical assembly.
2. Description of the Prior Art
An important problem encountered by those skilled in the art when developing a technology of lining metallurgical assemblies, e.g. induction furnaces consists in increasing stability of the lining with a simultaneous reduction of expenses required for manufacturing said lining.
The process of lining a metallurgical assembly is generally carried out as follows (see M. G. Trofimov, Futerovka induktsionnykh pechei, Moscow, "Metallurgia", 1968, pp. 129-132). First, a bottom is lined using a conventional method, following which a gauge for forming an inner wall of the future lining (a crucible) is mounted on said bottom. The space provided between the gauge and a corresponding element of the assembly, forming an outer wall of the lining (in the induction furnace this element is an induction heater) is filled with a free-flowing lining mass, e.g. with quartz sand containing binding additives. Following this the lining mass is compacted using various methods. The lining thus obtained is then sintered to increase its strength and resistance to the effect of a melt.
It should be noted that since the lining serves as a separating barrier between the melt and the cooled induction heater of the furnace, three zones having different degrees of sintering are present therein, the existence of these zones being caused by a relatively high temperature gradient in the direction of the thickness of said lining. The lining of the first zone (which is the closest to the melt) is the most sintered and the strongest one. The lining of the second (intermediate) zone, due to a lower temperature, is sintered to a lower degree than in the first zone and is less strong. In the third zone (abutting with the induction heater of the furnace) of the lining there is almost no sintering since individual grains of the refractory material are practically not bound between themselves.
In the course of lining the steps of filling and compacting the lining mass must be carried out in such a manner as to ensure:
(a) the highest degree of compaction in the first zone in order to obtain the minimum porosity and the maximum strength of the lining. These properties are necessary since the lining of this zone is to resist the effect of the melt and melting products;
(b) a lower degree of compaction in the second zone (and a higher porosity);
(c) the lowest degree of compaction in the third zone, i.e. the highest porosity since this zone is to be a buffer one, to provide for compensation of thermal expansion of the lining, and to lower impact effect exerted on said lining in the course of charging the furnace.
To increase the resistance of the lining of the assembly, the lining mass is to be uniformly compacted in the direction of the crucible height.
Moreover, in the course of lining the initial granular composition of the lining mass is to be maintained, i.e. fraction separation thereof must be eliminated.
In this connection it should be noted that the granular composition of the mass and distribution of grains over the volume of the lining mass influence the ratio between the volumes of closed and open pores and the total value of mass porosity, thereby determining numerous properties of the lining, and first of all strength and resistance to the effect of melt. The granular composition of the mass determines the number of contact points between the grains of the refractory material per unit of volume. With the optimum granular composition, voids between coarse grains are filled to the maximum extent with finer grains. The number of contact points and consequently density of the lining mass increase, thereby promoting an increase in the lining resistance.
It should be also noted that in the course of lining the local depletion or enrichment of the lining mass with a binder is to be eliminated.
With all the above requirements being met, the lining will possess high operation reliability.
In order to obtain uniform compaction of the lining mass with the crucible height, numerous prior art methods of lining provide layer-by-layer filling and compacting said mass. The step of compacting the layers is usually accomplished by ramming (USSR Inventor's Certificate No. 500,452).
The disadvantage of such a technology lies in the fact that in the course of ramming the lining mass is compacted non-uniformly along the crucible height, while along the thickness thereof the mass is uniformly compacted, due to which fact the third (buffer) zone of the lining, which must possess absorption properties, becomes excessively compacted. This results in decreasing the lining durability. Moreover, the step of ramming is a laborious and hard-to-mechanize operation, which results in a considerable increase in expenses for making the lining.
Also known in the art is a method of lining wherein, in order to obtain various properties in the direction of thickness of a lining, it has been proposed to use different lining masses and to fill them separately into a space provided between the gauge and the induction heater of the furnace, using a separating jacket (Swiss Patent Specification No. 476,272). According to this method, first the bottom of the furnace is lined, following which the gauge is mounted, and the jacket is placed between the gauge and the induction heater of the furnace, said jacket being constructed in the form of a thin-walled shell whose height does not exceed the diameter thereof. The jacket is fixed on three vertical helical rods mounted on the furnace body, said rods allowing the jacket to be either lifted or lowered relative to the bottom. The space between the gauge and the jacket and that provided between the jacket and the induction heater are filled with corresponding lining masses, the latter being subjected to compaction by ramming, shaking or vibratory compacting. In such a manner the first lining layer (in the direction of the crucible height) is formed. Following this, the jacket is lifted to a height corresponding to the thickness of the next layer, and the cycle is repeated. Using several such steps, the lining is made over the whole height of the furnace.
An obvious advantage of the above method of lining consists in the possibility of obtaining the lining having different zones with the crucible thickness, particularly two zones, and of using cheaper refractory materials for the outer (more distant from the melt) zone than those for the inner (more close to the melt) zone of lining.
In the practical realization of this method, however, there arise some serious difficulties. In particular, when compaction of the lining mass is accomplished by ramming, as required by an embodiment of the above technology, there arise difficulties similar to those accompanying the step of ramming in practicing other above described methods of lining.
When, in accordance with another embodiment of the invention, vibratory compaction is accomplished, the gauge will start vibrating and separation of the lining mass in accordance with the size of grains into separate fractions will occur, the coarse grains accumulating at the gauge. This results in an increase in the lining porosity within the first (inner) zone, thereby decreasing the resistance of the lining against the effect of the melt.
Moreover, with vibratory compacting the lining mass within the upper portion of the layer being compacted changes to a condition close to the fluidized one, which results in the local depletion or enrichment thereof with a binder and in a non-uniform compaction in the direction of crucible height. This phenomenon causes lamination of the lining and, as a sequence, penetration of the melt thereinto, which results in reducing service life of said lining.
It should be also noted that in the course of compacting the lining mass by means of various vibrators, the organism of a man carrying out lining operations is subjected to the harmful effect of vibrations.
Compaction of the lining mass, accomplished in accordance with the third embodiment of the above technology by shaking results in an increase in the total porosity of the lining (over the whole volume thereof) and as well as vibratory compaction, does not ensure uniform compaction of the mass in the direction of crucible height.
All the above difficulties inhibit wide practical application of said technology.